Generally, a passive optical network comprises an optical line terminal (OLT), a remote node (RN), and a plurality of optical network units (ONUs). A terminal such as a personal computer (PC) is connected to the ONU and the ONU will transform a signal transmitted from the terminal into an optical signal. The optical signal is split by an optical splitter at the RN and then transmitted to a central office (CO) of a service provider, i.e., the aforesaid OLT, through an optical fiber. After the OLT performs various types of signal processing, communication between one ONU and another ONU or communication between the ONU and another terminal on network can be carried out.
Currently, an optical access network can be classified into a time-division multiplexed passive optical network (TDM-PON), a wavelength-division multiplexed passive optical network (WDM-PON), and a hybrid passive optical network (Hybrid PON).
For TDM-PON, 10G-PON was accomplished and standardized in 2010 and next generation will be 40G-PON or 100G-PON. For 10G-PON, each concurrent user can have 10/N Gb/s bandwidth in average, where N is the number of ONUs. In another aspect, the WDM-PON is not standardized yet. Current technology can offer each user with 1.25 to 10 Gb/s bandwidth. The WDM-PON is a virtual point-to-point topology, which needs a pair of transceivers at the OLT for each ONU.
The TDM-PON structure may not serve the needs when the demand for larger bandwidth is increased. This is because it might be more difficult in system design and costly to use higher speed transceivers that are required in 40G-PON or 100G-PON. Especially, it might require burst mode transceivers for higher data rate. Also, power budget might be a problem. It may need avalanche photo-diode (APD) receivers, forward error correction (FEC) encoders, or optical amplifiers at the OLT or even at the ONUs. Further, it may eventually need to use cooled laser sources to avoid signal fluctuation caused by temperature variation.
The advantages of the WDM-PON structure is that it has larger bandwidth and gives much flexibility for different types of services and different bandwidths, and at the same time has better security. However, the WDM-PON structure is costly. If services are provided to N users, N dense wavelength-division multiplexing (DWDM) transceivers and N colorless ONU light sources are needed at the OLT. However, the DWDM transceivers and colorless light sources are still quite expensive. In addition, different wavelength bands may be needed for upstream and downstream transmission. For example, assuming that the channel spacing is 0.8 nm and the structure is designed to provide for 32 users, the total optical bandwidth in use is required to be 51.2 nm, and this occupies a quite large optical bandwidth.
Further, for the channel fault monitoring (CFM) issues for TDM-PON, special high-sensitivity OTDR (optical time-domain reflectometer) such as a photon-counting OTDR needs to be used for the monitoring due to large splitting loss of the optical splitter at the RN. Meanwhile, it is also proposed to add optical filters or wavelength-selective reflectors (e.g., fiber Bragg gratings, FBG) to the distribution fibers and use tunable OTDR (T-OTDR) to locate fiber breaks among the distribution fibers. The tunable OTDR is relatively high in cost and the use of the tunable OTDR requires a larger optical bandwidth for the channel monitoring.
For the channel fault monitoring issues for WDM-PON, a large optical bandwidth for the channel fault monitoring is needed because it requires an OTDR with a tunable light source or a broadband light source to reach each distribution fiber due to the wavelength selective characteristic of the WGR at the RN. For example, assuming that the channel spacing is 0.8 nm and the WDM-PON structure is designed to provide for 32 ONUs, the required optical bandwidth for the channel fault monitoring is 25.6 nm besides the possible 51.2 nm bandwidth for the upstream and downstream transmission. Optical bandwidth might be very tight if more service channels (e.g., video, audio, or radio over fiber (RoF)) are going to be added to the WDM-PON structure.
In addition, Hybrid PON generally has following two types: (1) cascading WDM-PON with a TDM-PON for extended services to more users and/or longer distance with an extended box; (2) connecting WDM-PON and TDM-PON in parallel to provide both point-to-point (i.e., WDM) and broadcasting (i.e., TDM) services.